1. Field
The present invention relates generally to magnetic devices, and more particularly, relates to high-density magnetic devices, such as magnetic memory and/or sensors, and methods of manufacturing such devices.
2. Related Art
For over 2000 years, magnetic devices have been beneficially deployed in navigation equipment for sensing the Earth's magnetic poles. Current magnetoelectronic devices may be used in medical applications for magnetic resonance imaging; in military surveillance for detecting submarines and buried landmines; in highway toll systems for traffic detection of vehicles for automated-toll-pay systems; in disk drives as magnetic pickup heads; in magnetoelectronics memories for Magnetic Random Access Memories (MRAM); and in automated industrial equipment for proximity sensors.
Magnetoelectronic devices may be used to measure the presence, magnitude, and/or direction of a magnetic field, changes in a magnetic field due to a presence of a ferromagnetic object, characteristics of the Earth's field, and electrical current flow. Many different types and constructions of magnetoelectronic devices exist.
The types and constructions are generally dictated by the sensing technology, and the detectable magnetic field. Accurately and reliably measuring magnetic fields smaller than the Earth's field may present an obstacle for many magnetoelectronic devices, and thus, may limit the type and construction of the magnetoelectronic devices. One such device that appears to overcome these obstacles is a magnetoelectronic device that employs the giant magnetoresistive (GMR) effect. Magnetoelectronic devices that employ the GMR effect may be capable of measuring small fields from magnetized objects, electrical currents, deviations in the Earth's magnetic field, and non-magnetized objects.
A. Giant Magnetoresitive Effect
Reportedly, as a result of recent advances in the art of thin-film material processing, the giant magnetoresistive (GMR) effect was discovered in 1988 by Baibich et al. The GMR effect describes the phenomenon of dramatic resistance drop in certain materials in the presence of magnetic fields. This change in resistance divided by the total resistance of the GMR device may be defined as the magnetoresistive (MR) resistivity sensitivity of the GMR device.
At the core of many GMR magnetoelectronic devices is a GMR sensor element. The GMR sensor element may be used as the foundation for GMR magnetoelectronic devices, including unpinned sandwiches, antiferromagnetic multilayers, and antiferromagnetic pinned spin valves. Generally, this GMR sensor element is constructed in a stack configuration in which the stack contains a number of deposited layers of thin-film materials. Common to most GMR sensor elements, the minimum number of layers in the stack usually includes three layers—two magnetic layers separated by at least one conductive nonmagnetic spacer layer. It is believed that the MR resistivity sensitivity of the tri-layer and other multilayer stacks is a function of the thickness of the stack's spacer layers and the phenomenon of spin-dependent scattering of conduction electrons at the boundaries between the spacer layers and the magnetic layers.
In the absence of an external magnetic field and with a given thickness of a spacer layer, the magnetic layers in a tri-layer stack configuration may exchange magnetic coupling. This coupling may oscillate between ferromagnetically coupling and antiferromagnetically coupling depending on the thickness of the spacer layer.
The antiferromagnetic coupling is believed to cause the magnetic moments of the two magnetic layers to become antiparallel. In this antiparallel state, the stack of materials comprising the GMR sensor element is believed to exhibit maximum spin-dependent scattering of conduction electrons. The maximum spin-dependent scattering of conduction electrons in turn is believed to place the GMR sensor element in a maximum resistance state.
By applying an adequate magnetic field to overcome the antiferromagnetic coupling, the antiparallel magnetic moments of the magnetic layers become parallel, thereby decreasing the spin-dependent electrons of the conduction electrons, and likewise, decreasing the resistance in the GMR magnetoelectronic devices. On the other hand, the ferromagnetic coupling is believed to cause the magnetic moments of the two magnetic layers to become parallel, which in turn is believed to exhibit something less than maximum spin-dependent scattering of conduction electrons or a lower resistance state.
B. Magnetoresistive Random Access Memory
As with other magnetoelectronic devices, the GMR structure of an MRAM consists of a GMR stack having a spacer layer “sandwiched” between two magnetic layers. This structure is normally manufactured using integrated circuit processing techniques by forming the GMR stack one sheet or layer of material over another and then subdividing the stack of layers into individual GMR stacks commonly referred to as “bits.” Each of the bits are binary. That is, they “store” or take on two discrete states, commonly denoted as a “0” or a “1.”
Given that the magnetic moment (or moments) of each of the magnetic layers may be selectively aligned along any axis, setting the “0” or “1” state may vary. In one option, the “0” state occurs by setting a magnetization vector (i.e., aligning the magnetic moment or moments) of the bottom magnetic layer or the “storage layer” of the MRAM bit in a horizontal plane with its direction pointing to the left. To sense or “read” that the MRAM bit is set to the “0” state, the change in resistance of the stack is measured as the magnetization vector of the top magnetic layer or the “sense layer” is flipped. For instance, by changing the magnetization vector of the sense layer from its default position, i.e. changing it to the parallel storage layer position, to a position antiparallel to the storage layer, the MRAM bit changes from low resistance to high resistance and indicates the MRAM bit is set to the “0” state.
On the other hand, the “1” state occurs when the alignment of the magnetization vector (i.e., the alignment of the magnetic moment or moments) of the storage layer of the MRAM bit is set in a horizontal plane with its direction pointing to the right. By flipping the magnetization vector top magnetic layer or the “sense layer” from its default position, which is now antiparallel (due to the change in direction of the storage layer), to parallel causes the resistance of the MRAM bit to switch from high resistance to low resistance, thus indicating the MRAM bit is in the “1” state.
For the MRAM bit to function, there must be a way to “write” the bit, i.e., change the bit from one state to another, and to “read” the bit, i.e., sense the bit's state. This may be done by using the same material for both the magnetic layers, but having different thicknesses. This causes the thicker layer to be more resistant to change in the presence of an externally applied magnetic field than the thinner layer. The magnetization vector of the thinner layer will change or “flip” at a lower field strength than the thicker layer. Accordingly, the storage layer of the GMR stack will be thicker than the sense layer.
Therefore, a small magnetic field (e.g., a current) may change only the sense-layer-magnetization direction, while a large magnetic field may change both the sense and storage layer magnetization-vector directions. Details describing the principals of operation of MRAM may be found in Tumanski. S., Thin Film Magnetoresistive Sensors, U.K., Institute of Physics, 2001, p. 353–357. These details are fully incorporated herein by reference.
C. Magnetoresistive Sensors
Like the magnetic memories, magnetoelectronic sensors function by sensing the change in resistance in the GMR stack. For example, a GMR stack deployed in a disk drive's read/write head reacts similarly to a magnetic memory. The GMR sensor stack in the disk drive heads, however, are typically comprised of four layers of thin material sandwiched together into a single structure. Generally, this structure includes the tri-layer GMR stack and an additional layer formed adjacent to one of the magnetic layers. The additional layer is known as the exchange or “pinning” layer. The exchange layer fixes or “pins” the magnetization of one of the magnetic layers, usually the adjacent magnetic layer, in the same direction as the magnetization of the pinning layer, thus causing the adjacent magnetic layer to become a “pinned” layer. The other magnetic layer or “free layer,” however, is free to change magnetization direction in the presence of a magnetic field.
In operation, when the disk drive head passes over a magnetic field of one polarity on a disk, which corresponds to a “0” state, the free layer changes from its default magnetization direction, i.e., antiparallel, to parallel. That is, the “0” state indicates that the GMR stack changes from a high resistance state to a low resistance state. When the head passes over a magnetic field of the opposite polarity or direction, i.e., a “1” state, the free layer magnetism rotates so that they antiparallel with the pinned layer.
Widespread application for these devices may reside in replacing existing non-magnetic circuitry performing similar functions. Given the non-volatility, and fast read/write performance of the GMR stack, these devices may become the product of choice. To do this, however, the magnetoelectronic devices should meet the size and performance of current devices. In manufacturing these magnetoelectronic devices, problems occur as the circuit density of GMR integrated circuits (“chips”) increases. Problems in processing high-density GMR chips may cause the manufacturing yield to fall to about zero, making the GMR chips technologically or monetarily impracticable. These processing problems may occur at various stages in manufacturing GMR chips. For instance, current manufactures and/or manufacturers of MRAM bits (and other GMR devices) use chromium-silicon (CrSi) films to act as an etch stop layer during bit formation. During the formation of the etch stop layer, the CrSi target manufacturing processes produce defects in deposited layers. When attempting to manufacture a megabyte GMR memory integrated circuit, calculations indicate that the defects occurring during the formation of the CrSi layer alone would reduce the yield to approximately 12%. When combined with all process steps, the ability to yield a functional GMR integrated circuit is near zero.
Therefore, it would be desirable to reduce defects so as to increase yield and to lower cost of manufacture in order to manufacture and provide high-density magnetoelectronic devices. Further, it would be desirable to improve the manufacturing of magnetoelectronic devices by simplifying processing steps, and increasing the repeatability of the processing steps.